The present application relates to color pixel arrangements, and specifically to color pixel arrangements used in electronic imaging devices and displays.
Full color perception is produced in the eye by three-color receptor nerve cell types called cones. The three types are sensitive to different wavelengths of light: long, medium, and short (“red”, “green”, and “blue”, respectively). The relative density of the three differs significantly from one another. There are slightly more red receptors than green receptors. There are very few blue receptors compared to red or green receptors. In addition to the color receptors, there are relative wavelength insensitive receptors called rods that contribute to monochrome night vision.
The human vision system processes the information detected by the eye in several perceptual channels: luminance, chromanance, and motion. Motion is only important for flicker threshold to the imaging system designer. The luminance channel takes the input from all of the available receptors, cones and rods. It is “color blind”. It processes the information in such a manner that the contrast of edges is enhanced. The chromanance channel does not have edge contrast enhancement. Since the luminance channel uses and enhances every receptor, the resolution of the luminance channel is several times higher than the chromanance channel. The blue receptor contribution to luminance perception is less than 5%, or one part in twenty. Thus, the error introduced by lowering the blue resolution by one octave will be barely noticeable by the most perceptive viewer, if at all, as experiments at NASA, Ames Research Center (R. Martin, J. Gille, J. Larimer, Detectability of Reduced Blue Pixel Count in Projection Displays, SID Digest 1993) have demonstrated.
Color perception is influenced by a process called “assimilation”, or the Von Bezold color blending effect. This is what allows separate color pixels (or sub-pixels or emitters) of a display to be perceived as the mixed color. This blending effect happens over a given angular distance in the field of view. Because of the relatively scarce blue receptors, this blending happens over a greater angle for blue than for red or green. This distance is approximately 0.25° for blue, while for red or green it is approximately 0.12°. At a viewing distance of twelve inches, 0.25° subtends 50 mils (1,270μ) on a display. Thus, if the blue pixel pitch is less than half (625μ) of this blending pitch, the colors will blend without loss of picture quality.
The present state of the art of color single plane imaging matrix, for flat panel displays and solid state camera chips is the (red-green-blue) RGB color triad. The system takes advantage of the Von Bezold effect by separating the three colors and placing equal spatial frequency weight on each color. Two manufacturers have shown improvements in display design by using dual or triple panels whose images are superimposed. One manufacturer of projection displays used three panels, red, green, and blue. The blue panel utilizes reduced resolution in accordance with the match between human vision requirements and the displayed image. Another manufacturer, Planar Systems of Beaverton, Oreg. employs a “Multi-row Addressing” technique having a dual electroluminescent panel, one panel with red and green pixels, the other with blue pixels to build a developmental model. The blue pixels have reduced resolution in the vertical axis only. This allows the blue phosphors to be excited at a higher rate than the red and green pixels, thus overcoming a problem with lower blue phosphor brightness. The problem with the prior art is that in providing the same matched resolution balance between human vision and display, additional display panels/planes are used, along with additional driver electronics.
Other display methods such as disclosed in U.S. Pat. No. 6,008,868 to Silverbrook use binary controlled emitters. In using binary controlled emitters, each emitter has a discrete luminance value, therefore, requiring the display to have an exact area to luminance relationship. This prior art used reduced blue “bit depth” built into the panel in accordance with human vision's lower blue color space increments. Conventional display methods also use a single color in a vertical stripe. Since conventional stripes have limited the Modulation Transfer Function (MTF), high spatial frequency resolution, in the horizontal axis, stripes of a single color are non-optimal.
Display devices can include liquid crystal display (LCD) devices. LCD devices have been used in a variety of applications, including calculators, watches, color televisions, and computer monitors. A conventional liquid crystal panel typically includes a pair of transparent glass substrates that are arranged in parallel to define a narrow gap therebetween that is filled with a liquid crystal material. A plurality of pixel electrodes typically are disposed in a matrix on an inner surface of one of the transparent glass substrates, and a plurality of common electrodes corresponding to the pixel electrodes are arranged on the inner surface of the other substrate of the two transparent glass substrates. A liquid crystal cell is defined by opposing pixel electrodes and common electrodes. Images are displayed by controlling light transmission through the cell according to a voltage applied to the electrode pair.
In a conventional active matrix LCD device, a plurality of row lines are formed on one substrate, transverse to a plurality of column lines. A plurality of pixel electrodes are disposed on a corresponding plurality of pixels regions defined by the row and column lines. A respective thin-film transistor (TFT) is formed on a respective one of the pixel regions, and drives the pixel electrode formed thereon.
Repeatedly driving a liquid crystal cell with voltages having the same polarity can cause an electrochemical change in the pixel electrode and the common electrode due to migration of ionic impurities within the liquid crystal material. This change can significantly reduce display sensitivity and brightness. Accordingly, it is generally desirable to repeatedly invert the polarity of the voltage applied to the liquid crystal cell in order to prevent this phenomenon. This method of driving a liquid crystal cell is known as “inversion”. There are several inversion schemes that are known in the art, including “frame inversion”, “column inversion”, “line (or row) inversion”, or “dot inversion”.
A conventional dot inversion driving technique involves applying column line voltages that have different polarities to adjacent sub-pixel electrodes, for example, by driving alternating pixel elements with negative and positive voltages. Typically, the polarity of the driving voltage applied to a given pixel electrode is inverted each time the voltage is applied. The applied voltage is stored on the sub-pixel, row by row, alternating with each row. The result is a “checker board” pattern of polarities on the two dimensional matrix of sub-pixels.
Although the above-mentioned conventional dot-inversion driving technique is useful to prevent ion migration in the liquid crystal material and lowering perceived “flicker” in the display. Special care must be taken when applying “dot inversion” to the novel arrangement of the three-color pixel elements, and its associated drive structure to avoid this “flicker”.